Bypass steam line

ABSTRACT

A mixing unit for mixing water with steam in a bypass station is provided. The mixing unit has a plurality of Laval nozzles arranged in the mixing unit, which Laval nozzles are displaced axially with respect to one another in a water steam direction, with the result that the noise emissions are reduced overall.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2012/065121 filed Aug. 2, 2012, and claims the benefitthereof. The International Application claims the benefit of EuropeanApplication No. EP11179513 filed Aug 31, 2011. All of the applicationsare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a mixing unit for mixing a flow medium with acooling medium, having a pipe conduit section, to which a mixing sectionis coupled fluidically, the mixing section having a plurality of Lavalnozzles through which the flow medium can flow, there being formed inthe Laval nozzles injection ducts through which the cooling medium flowsin such a way that mixing of the flow medium with the cooling mediumtakes place.

BACKGROUND OF INVENTION

In steam power plants, steam is generated in a steam generator whichconverts the thermal energy of the steam into rotational energy in aturbo set coupled fluidically to the steam generator. The rotationalenergy is finally converted into electrical energy. As long as the steampower plant operates continuously and the load on the electricalgenerator is comparatively constant, the thermal dynamic conditions arecomparatively constant over time.

There are situations, however, in which the steam power plant has to beadapted to rapidly changing load situations. It may be, for example,that an incident occurs and the generator suddenly has to be separatedfrom the network. It may also happen that the steam power plant has tochange over from full load to part load unpredictably. Such load changesare a challenge to the technology for regulating the overall steam powerplant. One possibility for following or counteracting rapidly changingload situations is to route the steam generated by the steam generator,and flowing directly to the high-pressure subturbine during continuousoperation or full-load operation, directly to the condenser via a bypassstation. In this bypass station, devices are provided, which mix thehighly heated steam with water, in order thereby to change thethermodynamic conditions of the steam. This water is injected into thesteam. According to the prior art, this takes place in a bypass stationin which is arranged a Laval nozzle having injection ducts through whichwater is sprayed into the steam.

It has been shown, however, that, because of this, the noise emission iscomparatively high. Moreover, it has been shown that the temperaturedistribution is not sufficiently homogeneous, thus leading to anon-optimal operating behavior under part load.

Bypass stations used nowadays are composed essentially of a bypass valveand of the bypass steam infeed. The bypass steam infeed comprises adiaphragm, a water injection device and a mixing pipe. When the steampower plant is started up or after a trip, steam occurring in the steamturbines is cooled via the bypass station by the injection of water andis introduced directly into the condenser.

SUMMARY OF INVENTION

An object herein is to allow better mixing of the water with the steamduring operation and at the same time to reduce noise emission.

This object is achieved by a mixing unit for mixing a flow medium with acooling medium, comprising a pipe conduit section, to which a mixingsection is coupled fluidically, the mixing section comprising aplurality of Laval nozzles, through which the flow medium can flow,there being formed in the Laval nozzles injection ducts through whichthe cooling medium flows in such a way that mixing of the flow mediumwith the cooling medium takes place, Laval nozzles adjacent to oneanother being arranged so as to be offset in relation to one another inthe direction of flow of the flow medium.

An aspect of the invention thus pursues the path of using a plurality ofdiaphragm orifices, contrary to the existing concept in which the steamflows through only a single diaphragm orifice. The disadvantage arisingfrom the use of a single diaphragm orifice is that mixing is notoptimal, particularly at the margins of the mixing section. Bettermixing and reduced noise emission are achieved, using a plurality ofdiaphragm orifices.

An aspect of the invention is that two Laval nozzles adjacent to oneanother are arranged so as to be offset in relation to one another inthe direction of flow. To cool the steam, in bypass mode water isinjected into the bypass steam line. In order to achieve goodatomization of the water and therefore effective cooling, the steam,before being admixed, is routed through a Laval nozzle or through aperforated diaphragm, with the result that the flow velocity risessharply. The high relative velocity between the steam and the waterdrops leads to good atomization, but has the disadvantage that the waterdrops do not reach into the core of the steam stream and therefore theinner part or the inner core of the steam stream is not sufficientlycooled. The Laval nozzles are in this case displaced axially in thedirection of flow of the flow medium. When a flow passes through a Lavalnozzle, a sound wave is generated. The sound wave arises behind a Lavalnozzle. If Laval nozzles are additionally displaced axially with respectto one another by a length, so that sound wave peaks and sound wavetroughs of different Laval nozzles adjacent to one another cancel eachother out, the overall sound emission is markedly reduced.

A feature here, therefore, is that the individual Laval nozzles whichare arranged adjacently to one another are mutually displaced axially.

The Laval nozzles are coupled to a displacement device, displacement ofthe Laval nozzles being possible during operation. It is thus proposedto provide active displacement which can take place electrically orhydraulically or by other means, so that the Laval nozzle planes can bedisplaced with respect to one another in such a way that differentfrequency bands can be influenced during operation. Noise emission canthus be actively reduced in different operating states.

In a first advantageous development, the Laval nozzles are designedidentically to one another. This leads to better computability of thesound wave troughs and sound wave peaks, and it can thus bepredetermined more effectively by computations from the sound emissionhow far axial displacement has to take place.

In a further advantageous development, the Laval nozzles are coupled toa displacement device, displacement of the Laval nozzles being possibleduring operation. It is thus proposed to provide active displacementwhich can take place electrically or hydraulically or by other means, sothat the Laval nozzle planes can be displaced with respect to oneanother in such a way that different frequency bands can be influencedduring operation. Noise emission can thus be actively reduced indifferent operating states.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, then, is explained in more detail by means of anexemplary embodiment. In the drawings:

FIG. 1 shows a cross-sectional view of a conventional mixing unit;

FIG. 2 shows a cross-sectional view of a mixing unit according to theinvention;

FIG. 3 shows a cross-sectional view of part of the mixing unit.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a mixing unit 1 according to the prior art. Such a mixingunit 1 is characterized by a pipe conduit section 2 in which a flowmedium 3 flows in the direction of a mixing section 4. In this mixingsection 4, a Laval nozzle 5 is arranged, in which the flow medium isaccelerated. Arranged in the Laval nozzle 5 are injection ducts 6through which a cooling medium, such as water, flows. The cooling mediumis mixed with the flow medium 3 in a pipe section 7 which is connectedfluidically to the mixing unit 4.

FIG. 2 shows an illustration according to the invention of the mixingunit 1. The difference from the mixing unit 1 according to FIG. 1 isthat, in the mixing section 4, a plurality of Laval nozzles 5 a, 5 b, 5c are arranged, through which the flow medium 3 flows and in each ofwhich is formed an injection duct 6, by means of which water is mixedwith the flow medium. Furthermore, the difference between the mixingunit 1 of FIG. 1 and that of FIG. 2 is that the Laval nozzles 5 a, 5 b,5 c are displaced with respect to one another in the flow mediumdirection 8, which may also be designated as the axial direction. As aresult of this displacement, the sound wave troughs, which coincide withsound wave peaks of the adjacent Laval nozzles, are cancelled. Anoverall reduction in sound emission is thereby achieved. Finally, thepipe section 7 is connected to a condenser, not illustrated in any moredetail.

The axial displacement of the Laval nozzles 5 a, 5 b, 5 c with respectto one another may take place by active displacement by means ofelectrical or hydraulic forces. This may take place during operationwhere different operating states arise. Different frequency bands canthereby be influenced, thus reducing noise emission, overall, evenduring operation.

The frequency band can be measured during operation and the diaphragmscan then be displaced with respect to one another such that noiseemission becomes minimal. The most favorable axial positions can bedetermined beforehand for each load point during the commissioning ofthe plant, and these can then simply be input during operation, withoutthe frequency spectrum having to be measured actively.

FIG. 3 shows by way of example an illustration of the displacement ofthe Laval nozzles 5 a and 5 b. The Laval nozzle 5 a is displaced withrespect to the Laval nozzle 5 b by the length L. With a sound frequencyof 1000 Hz, this would give a sound velocity of approximately 500 m/s,thus resulting in the required length of 0.5 m. This length may be setstatically in the first approximation or, as described further above,may be obtained, even during operation, by active displacement.

1. A mixing unit for mixing a flow medium with a cooling medium,comprising a pipe conduit section, to which a mixing section is coupledfluidically, the mixing section comprising a plurality of Laval nozzles,through which the flow medium can flow, injection ducts formed in theLaval nozzles through which the cooling medium flows in such a way thatmixing of the flow medium with the cooling medium takes place, whereinthe Laval nozzles are adjacent to one another and arranged to be offsetin relation to one another in the direction of flow of the flow medium,and wherein the Laval nozzles are coupled to a displacement device,allowing for displacement of the Laval nozzles during operation.
 2. Themixing unit as claimed in claim 1, wherein the Laval nozzles aredesigned identically to one another.
 3. The mixing unit as claimed inclaim 1, wherein the injection ducts are formed obliquely to the Lavalnozzle wall.
 4. The mixing unit as claimed in claim 1, wherein the flowmedium comprises steam.
 5. The mixing unit as claimed in claim 1,wherein the cooling medium comprises water.
 6. The mixing unit asclaimed in claim 1, wherein displacement takes place electrically. 7.The mixing unit as claimed in claim 1, wherein displacement takes placehydraulically.